It is often required in many industrial fields to accurately and reliably measure the angular distance moved as well as instantaneous angular velocity of a rotatable shaft attached to, for example, a motor or a gear. Especially, these requirements are found in a precision-machine-related field, so that high accuracy as well as high reliability are indispensable. As the most desirable measuring devices to meet such requirements, several proposals based on digital control have been made heretofore.
One example of conventional products of this kind is the optical rotary encoder comprising, a glass disk with a plurality of photoslits, one or more LEDs (light emitting diodes), and one or more photo-sensitive diodes. The photo-slits are usually provided on the glass disk by the following steps: firstly, a suitable metal film is plated on the disk by vapor deposition techniques, secondly photo-resist film is coated on the metal film and is selectively exposed to light and then developed, and finally the areas not exposed are chemically etched. The LED and the photo-diode are positioned at the opposite sides of the glass disk for detecting the rotating conditions of the disk.
However, this optical encoder involves some difficulties: (1) it requires a highly accurate exposure as well as highly skilled etching techniques to provide the photo-slits on the entire surface of the disk without aberration, (2) it is practically very difficult to accurately adjust the center of the photo-slits with that of a rotatable shaft receiving bore, (3) in order to tightly receive the rotatable shaft, such a bore should be precisely processed and finished, (4) as a consequence the encoder costs too much for achieving initially intended results, and (5) furthermore, the encoder is vulnerable to external shocks.
Meanwhile, a magnetic tachometer was proposed in IBM Technical Disclosure Bulletin, Volume 16, No. 1, (June 1, 1973) page 260. The tachometer comprises a disk carrying a magnetic medium on one surface near its perimeter, and a magnetic flux transducer. The transducer senses magnetic signals recorded on the magnetic medium to determine the rotational speed of the disk and, therefore the rotational speed of the shaft to which the disk is connected. However, the above-mentioned article merely suggests the possibility of its realization, without giving a detailed description.
Another example of a conventional magnetic rotary encoder is disclosed in the U.S. Pat. No. 4,039,936. This encoder comprises a plurality of thin plate-like permanent magnets, a rotatable drum having a shaft, and a magnetic field sensing transducer. The permanent magnets are provided on the outer perimeter of the drum with every two adjacent ones given opposite directions of magnetization. The magnetic field sensing transducer is fixedly positioned near the rotating paths of the magnets. It is acknowledged that this prior art has some advantage: it withstands external shocks; and the drum can be precisely manufactured in dimensions so that it is free from undersirable mechanical vibrations. It is, however, unsuitable for directly measuring absolute values of angular displacement through which the shaft turns.
An object of the present invention is therefore to provide an improved magnetic rotary encoder for precisely and reliably measuring in abosolute value the rotated angular distance as well as angular velocity of a rotatable shaft.
Another object of this invention is to provide an improved magnetic rotary encoder, which can be precisely manufactured, accurately installed, and has a high shock-resistant property.